1. Field of the Invention
The present invention relates generally to antennas. More specifically, the present invention relates to a fan shaped conical antenna that is suitable for use in a direction finding array used in a direction finding system. Instead of using a full circular cone, the antenna includes only a sector of a complete circular cone. The antenna is used in a direction finding array of antennas that operates in the VHF/UHF/SHF bands.
2. Description of the Related Art
Conical antennas, which include a single inverted cone over a ground plane, and biconical antennas, which include a pair of cones oriented with their apexes pointing toward each other are used as broadband antennas for various applications, including direction finding. FIG. 1 is a schematic diagram illustrating two such antennas. A biconical antenna 100 includes a top inverted cone 101a and a bottom cone 101b. An electronic coupler 102 provides a connection to a feeding circuit (not shown) that provides an electrical signal that feeds the antenna. It should be noted that the antenna is symmetric about the cone axis and that each of the cones is a full cone, spanning 360xc2x0. Similarly, a single cone antenna 110 includes a single antenna cone 111 that also spans 360xc2x0 and is symmetric about the cone axis. Single antenna cone 111 is connected to an electronic coupler 114 that provides a connection to a feeding circuit (not shown) that provides an electrical signal that feeds the antenna. The single cone antenna is located over a ground plane 112.
Antennas such as the ones shown above may be included in an array of antennas used for direction finding. Direction finding antenna arrays determine direction by comparing the phase or strength of signals received at different antennas. According to the principle of reciprocity, signals may likewise be sent from an array in a particular direction by altering the phase or strength of the electric signals feeding each of the antennas. Hereinafter, antenna surfaces such as top inverted cone 101a and bottom cone 101b are referred to as radiators and it should be understood that the antenna surfaces may be used to either radiate or receive a signal.
Because the conical antennas shown in FIG. 1 are symmetric about the cone axis, the radiation pattern from the antenna is omniazimuthal or isotropic with respect to the azimuth angle. As a result, the radiation patterns from such antennas tend to interfere with each other when a group of such antennas are included in an array. This complicates direction finding and may reduce the accuracy that may be achieved. Furthermore, each of the antennas take up a relatively large amount of space and must be spaced apart in an array to ensure that they do not physically interfere with each other.
What is needed is a direction finding antenna and a direction finding array made up of broadband antennas that do not radiate isotropically with respect to the azimuth angle and which occupy less physical space than the full conical antennas shown in FIG. 1.
Accordingly, the present invention provides a broadband antenna suitable for use in a direction finding array. In one embodiment, the antenna is designed for use over a 16:1 frequency bandwidth. The antenna uses only a sector of a circular cone and therefore occupies less space than a full cone. In a circular DF array, the antenna reduces the interactions between elements compared to typical omnidirectional elements. Asymmetries in the azimuthal antenna pattern of the antennas located along the perimeter of the array enables direction information to be determined from the amplitude of the signals detected by the antennas in the array. This provides more accurate and reliable direction finding.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium. Several inventive embodiments of the present invention are described below.
In one embodiment, a broadband partial fan cone direction finding antenna and array disclosed. The antenna includes a radiator having a partial cone shape. The radiator substantially occupies a spatial area defined by a portion of a cone and the cone is defined by a cone axis, a cone height, and a cone angle. The cone has a base and an apex, and the portion of the cone is defined by a cone sweep angle. The cone sweep angle is determined as the angle subtended by a projection of the portion of the cone projected onto a plane that is perpendicular to the cone axis. The cone sweep angle is less than 360xc2x0 so that the portion of the cone is bounded on its sides by edges which extend radially from the apex of the cone outward to the base of the cone.
In another embodiment, a flat fan antenna is disclosed that includes a flat metal radiator. The flat metal radiator is formed in the shape of a sector of a flat disc. The sector is defined by a sector angle, an inner radius and an outer radius. The sector angle determines the angle subtended by the sector; the inner radius determines the inner edge of the sector; and the outer radius determines the outer edge of the sector. The flat metal radiator is tilted from the vertical by a tilt angle. The flat fan antenna has a radiation pattern and performance similar to the pattern and performance of a partial fan cone antenna.
In another embodiment, An antenna direction finding array includes a plurality of direction finding antennas approximately spaced around the circumference of a circle. Each direction finding antenna is pointed outward from the center of the circle, and each of the direction finding antennas has an amplitude response that is greater in the azimuthal direction that the antenna is pointed. A central reference antenna has an amplitude response that is substantially omniazimuthal so that the central reference antenna is suitable to be connected to a reference receiver and a signal from the central reference antenna is suitable for use as a reference for measuring the phase of the signals from the plurality of direction finding antennas.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.